The present invention relates to a process for producing aqueous emulsions of vinyl chloride/vinyl ester/ethylene copolymer having an improved heat fluidity.
It is well known that aqueous emulsions of a copolymer of vinyl chloride, a vinyl ester (e.g. vinyl acetate) and ethylene have excellent properties such as excellent water resistance and alkali resistance and are widely used as an adhesive, paint and a binder for processing of fibers and papers. In order to improve the properties such as water resistance, flame retardant properties and chemical resistance of these aqueous emulsions of a vinyl chloride/vinyl acetate/ethylene copolymer, the amount of vinyl chloride to be copolymerized may be increased. However, when the amount of vinyl chloride is increased, the resulting copolymer of vinyl chloride/vinyl acetate/ethylene has a lower molecular weight and the aqueous emulsion of the copolymer shows undesirably a large heat fluidity. For instance, when the aqueous emulsion of a vinyl chloride/vinyl acetate/ethylene copolymer having a lower molecular weight is used as an adhesive for a thermoplastic resin and the resulting thermoplastic resin product is subjected to embossing, the emboss is easily deformed by subjecting the product to heat. Moreover, when the aqueous emulsion is used as a vehicle of a paint composition, the coating film obtained therefrom has an inferior strength, for example, an inferior washing resistance. Furthermore, when the aqueous emulsion is used as a binder for fibers, the fibers are inferior in heat stability because of the large heat fluidity due to the copolymer.
It is known that a polyfunctional monomer having two or more polymerizable double bonds in the molecule can be introduced into a copolymer in order to increase the molecular weight of the polymers or copolymers and that the polyfunctional monomer is used as a cross linking agent, and thereby, a gel insoluble in an organic solvent can be produced. For example, it is disclosed in Japanese Patent Publication No. 37012/1977 that an aqueous emulsion of vinyl acetate/ethylene copolymer is produced by using a vinyl ester of a polybasic acid or an allyl ester of a polyfunctional acid as a cross linking agent in order to improve the solvent resistance or mechanical properties at a high temperature. According to re-examination of this process by the present inventors, however, when triallyl cyanurate as used in the working example of this Japanese patent was used as a cross linking agent, the resulting copolymer had an increased amount of benzene-insoluble part as desired, but the aqueous emulsion of the copolymer contained a large amount of coarse particles and produced a large amount of precipitates during storage thereof, that is, the aqueous emulsion was very unstable.
Furthermore, it is disclosed in Japanese Patent Publication No. 19179/1971 that when divinyl succinate or glycol dimethacrylate is used in an amount of up to 1% by weight (based upon the total weight of the monomers) in the preparation of an aqueous emulsion of vinyl chloride/vinyl acetate/ethylene copolymer having a vinyl chloride content of 35% by weight or less, the molecular weight of the copolymer is increased and a cross linked copolymer can be produced. It is also disclosed in Japanese Patent Publication No. 9671/1967 that saturated hydrocarbon diol divinyl ethers or dibasic acid diallyl esters can be used for producing a polyvinyl chloride having a high degree of polymerization and having low heat deformation properties and an excellent processability without producing a large amount of gel which is insoluble in an organic solvent. However, neither improvement of the properties of vinyl chloride/vinyl ester/ethylene copolymer nor increase of the amount of benzene-insoluble part nor increase of the molecular weight could be achieved even when glycol dimethacrylate or glycol diacrylate as disclosed in Japanese Patent Publication No. 19179/1971 or octadecandiol divinyl ether or trifunctional monomer triacrylate or trimethacrylate as disclosed in Japanese Patent Publication No. 9671/1967 was used in an amount of up to 1% by weight based upon the total weight of the monomers.
Moreover, there are disclosed in Japanese Patent Publication No. 32787/1974 unsaturated carboxylic acids, acrylonitrile, nitrogen-containing unsaturated compounds, allyl compounds (e.g. diallyl phthalate) and divinyl compounds (e.g. divinylbenzene) which may participate in copolymerization in the production of aqueous emulsions of copolymers of olefins, vinyl halides, vinyl esters and alkyl acrylates or alkyl methacrylates.
However, not all polyfunctional monomers containing two or more polymerizable double bonds in the molecule can be necessarily used for increasing the molecular weight of polymers or copolymers and for producing an organic solventinsoluble part by cross linking of the copolymer. Even if they can provide the desired polymer or copolymer, the aqueous emulsions thereof are occasionally unstable. Accordingly, careful attention should be paid to selection of the most suitable polyfunctional monomers depending on the kinds and the desired properties of the polymers or copolymers and also selection of the most suitable usage of the monomers.